The present invention relates to a process for manufacturing a coated, monolithic metal support formed of a volumetric curved, shaped body which is gas-permeable in the axial direction. Typically, the shaped body is cylindrical and is made up of one or more smooth and/or corrugated metal foil strips joined together by soldering or welding, their longitudinal extension lying transverse to the cylinder axis. The strips also optionally have slots, perforations and/or impressions and are stacked or folded and optionally intertwined or wound in a spiral manner in order to form the shaped body. In particular, the invention relates to a process for the manufacture of a monolithic metal support coated with a dispersion coating for use as a catalyst in chemical conversion reactions or as an absorber or adsorber for gaseous or liquid components of a mixture.
Coated metal supports are used in large numbers as catalysts in motor vehicle exhaust purification systems or as regeneratable absorbers or adsorbers for hydrocarbons and nitrogen oxides in exhaust gases.
The shaped bodies formed from metal foil strips have a cellular structure which permits the exhaust gas to be cleaned as it passes through in the axial direction. In the simplest case, the cellular structure consists of axially parallel flow channels. According to the type of corrugation of the metal foil strips, the flow channels may be inclined relative to the axial direction and also be angled. Slots, perforations and deformations give rise to a gas exchange between the various flow channels and to strong turbulence, and hence to intensive contact of the exhaust gas with the catalytically active coating.
Typical cell densities of metal support catalysts in vehicle exhaust catalysis are between 15 cm.sup.-2 (96 inch.sup.-2) and 78 cm.sup.-2 (500 inch.sup.-2). To improve the level of purification of the exhaust gases, cell densities of up to 186 cm.sup.-2 (1200 inch.sup.-2 ) are aimed at.
DE 29 46 685 C2 discloses a process for the manufacture of a metal support catalyst from a composite body disposed in a metal jacket. The composite body is made from smooth and corrugated or folded sheets of heat-resistant metal placed alternately one on top of another; the sheets being coated to form a bundle or wound up in a spiral manner, to produce a preliminary form of the composite body. This preliminary form is then welded at its ends, and the composite body is inserted into the metal jacket and is also welded thereto. Then the interior of the composite body is coated with a catalysis-promoting metal oxide.
DE 29 24 592 A1 also discloses a process for the manufacture of a support matrix for a metal support catalyst. The support matrix consists of smooth and corrugated sheets arranged alternately in layers and soldered together as a whole or by spot-soldering. The solder is applied to the specified places as a solder paste, a solder powder or a solder strip. For soldering, the whole support matrix is heated to the soldering temperature in a vacuum or in a protective gas atmosphere.
Both processes have in common that the catalytically active dispersion coating is not applied until the shaped body has been joined together, i.e. after welding or soldering. This means that, due to surface stresses, the dispersion coating collects at the acute angles between two contacting metal strips and thus the coating density over the cross-section of the shaped body is very uneven. The material collecting in the corners is not easily accessible to the exhaust gas to be cleaned, so that the catalytically active substance is not satisfactorily exploited. Furthermore, the accumulation of coating material also increases resistance to the flow of exhaust gas.
These disadvantages are less prominent in shaped bodies with comparatively low cell densities of up to 32 cm .sup.-2. At higher cell densities, however, retrospective coating does carry an increasing risk of narrowing and blockage of the cells, so that shaped bodies with cell densities of more than 93 cm.sup.-2 can barely be coated.
WO 92/14549 proposes to manufacture the shaped bodies from ready-coated metal foil strips at cell densities in the range between 800 and 1200 inch.sup.-2 (corresponding to 125 to 186 cm.sup.-2) and to insert the strips in the casing tube without further joining processes. In these metal support catalysts, adequate stability and resistance to pushing of the shaped body out of the casing tube due to exhaust pressure is only ensured by the high cell densities used. Metal support catalysts manufactured in this manner having relatively low cell densities do not have sufficient strength to withstand the pulsating exhaust gas pressure and the axial and radial oscillations caused thereby during operation.